1. Field of the Invention
The present invention relates to a device for measuring the density of a body, in particular a human body, under normal living conditions without being subjected to any restrictions.
2. Description of the Prior Art
As used throughout the application, the term "density" is taken to refer to the average density of an organism at a given moment under given physiological conditions, e.g., before or after inhalation, the consumption of a meal, etc.
The prior art in this domain does not easily lend itself to definition. It has been stated by some physicians that knowledge of body density is useful in principle, both for physiological research and pathological characterization, however, the devices developed for such measurements have encountered various difficulties until this time.
Direct measurement by the displacement of an equivalent volume of water not only calls for an artificial respiratory device but also entails discomfort caused to the patient by the total immersion which is necessary. Indirect measurement by the combination of two independent measurements, that of the volume of the human body and that of its mass, have likewise hitherto proven impractical for reasons which will now be set forth.
Known techniques for measuring the volume of a body comprise the steps of placing the body in an enclosure filled with a certain gas at a given pressure and of "treating" this gas by one of the following processes:
(1) Exciting a resonant acoustic frequency, which is a function of the residual volume between the enclosure and the body. This phenomena, adequately described by HELMHOLTZ, after whom such "resonators" have been named, is expressed by a relatively complex law in which the volume and the frequency are interdependent while at the same time being a function of additional variables.
(2) Subjecting the residual volume between a chamber and the body within the chamber to a known variation of volume and measuring the resulting pressure variation. The method may be "static", i.e., based on the use of one single compression level, or "dynamic", i.e., with compression varying according to an alternating function.
The above techniques are not as simple as they may appear. The possibilities offered by the first method are seriously limited by the necessity of preventing leaks from the chambers and also by the temperature fluctuations which occur. The problems of "propagation", i.e., the finite speed at which a disturbance in a gas is propagated, set limits to the possibilities offered by the second method.
Thus, whether the static or the dynamic method is adopted, the difference between isothermic and adiabactic compression has to be taken into account and the the coefficient .gamma., equal to the quotient obtained from the two specific heats, that prevailing at constant pressure and that prevailing at a constant volume, must be taken into account. Furthermore, it is necessary to accurately determine not only the volumetric variation creating the phenomenon, but also the pressure prevailing in the enclosure which plays a direct role in the measurement.
Whether the resonance method or the volume-pressure sounding method is used, the living organism being studied must not be injured or disturbed, a condition which neither of the techniques has been able to satisfy thus far.
Finally, whatever the volumetric measuring method adopted, the weight of the subject must still be determined as well. When independently measuring weight in order to determine density by simple division, a number of problems are encountered. There are, therefore, problems of not only convenience but also of rapidity, particularly in the case of physiological experiments, in which the weight of the subject may vary in the course of one and the same cycle of measurements and in which it must be possible for the density (varying as a result of the consumption of liquids or solids, urination, etc.) to be followed continuously.